Act I
The place: A mansion on Sunset Boulevard, Los Angeles, 5 a.m. Homicide has been reported. Joe Gillis sets the scene ("Prologue"), noting that it involves "an old-time movie star / Maybe the biggest star of all", and that, if you want to know the "real facts", you've "come to the right party."

Flashback to... Hollywood, 1949 - where a down-on-his-luck screenwriter, Joe Gillis, is trying to hustle up some work at Paramount Studios ("Let's Have Lunch"). His appointment with a producer goes poorly when the executive rejects both Joe's proposed script and a loan to bring his car payments up to date. Joe does, however, meet Betty Schaefer, a pretty, young script editor who suggests they collaborate to rework one of his earlier screenplays ("Every Movie's a Circus"). As they chat, car repossession agents spot Joe, who quickly escapes.

During the ensuing chase, Joe evades his pursuers by pulling into the garage of a palatial but dilapidated mansion on Sunset Boulevard. Beckoned inside the house, Joe encounters Norma Desmond ("Surrender"), a star actress of the silent-film era. Taken aback, Joe comments, "You used to be in pictures; you used to be big," to which Norma retorts, "I am big – it's the pictures that got small!" ("With One Look")

The gloomy estate is inhabited only by Norma and Max von Mayerling, her loyal butler and chauffeur. Although decades past her prime and mostly forgotten by the public, Norma is convinced she is as beautiful and popular as ever. Max perpetuates this illusion by shielding her from the realities of life out of the limelight and by writing her letters purportedly from still-devoted fans. Norma informs Joe that she plans to make her comeback with Salome, a script she has written for Cecil B. DeMille to direct with her in the starring role as the teenage biblical temptress ("Salome"). Dubious but sensing opportunity, Joe accepts her offer to work on editing the script. Norma insists that Joe stays in her home while they collaborate on Salome ("The Greatest Star of All").

Joe immediately realizes the script is incoherent, but because Norma won't allow a major rewrite, the revision drags on for months. During this time Joe is virtually imprisoned within the house, but he does break away to fulfill his commitment to Betty. Their working relationship blossoms into a romance that has her reconsidering her engagement to Joe's best friend, Artie Green ("Girl Meets Boy").

Blind to Joe's opportunism, Norma lavishes him with gifts that include a wardrobe makeover and he becomes her kept man ("The Lady's Paying"). She declares her love for him and turns quite possessive ("The Perfect Year"); when he leaves her to attend Artie's New Year's Eve party ("This Time Next Year"), she is distraught and attempts suicide. As a conciliatory gesture, Joe reluctantly returns to work on Salome.

Act II
Joe is now living in luxury at Norma Desmond's mansion, for reasons he bluntly states are mercenary ("Sunset Boulevard"). A cryptic message from Paramount has Norma certain that DeMille is eager to discuss her script ("There's Been A Call"). She drops in on the set of his current film and is greeted warmly by former colleagues and the famed director himself, but he is non-committal about Salome ("As If We Never Said Goodbye"). Meanwhile, Max discovers the studio had called to ask about Norma's Isotta Fraschini, not her screenplay. However, a delusional Norma leaves the lot convinced she'll soon be back in front of the cameras and begins to prepare for the role ("Eternal Youth Is Worth a Little Suffering").

Increasingly paranoid, Norma deduces that Joe and Betty are more than just friends ("Too Much in Love to Care"). She calls Betty to reveal Joe's secret life at the mansion, but he overhears and grabs the phone to tell Betty to come to see for herself. Realizing their affair is doomed, Joe brusquely tells her he enjoys being Norma's pet and that she should go back to Artie. Betty departs, confused and brokenhearted, and Joe tells Norma he is leaving her and returning to his hometown of Dayton, Ohio. He also bluntly informs her that Salome is an unfilmable script and her fans have long abandoned her. Furious and grief-stricken, Norma fatally shoots Joe three times as he storms out of the house.

Now completely insane, Norma mistakes the swarms of police and reporters who arrive for studio personnel. Imagining herself on the set of Salome, she slowly descends her grand staircase and utters "And now, Mr. DeMille, I am ready for my close-up."